Pipe in an oil or gas field or even in a gas or oil pipeline is typically joined by tapered screwthreads. In addition the joint is provided with extra formations intended to ensure a perfect seal without reliance on the screwthreads.
In German patent No. 1,533,619 (citing German patent document Nos. 1,152,673 and 1,243,605 and German utility model No. 1,906,009) filed Apr. 28, 1967 by H. Hillemanns et al the male part of the joint has a barrel-shaped outer end. Ihe corresponding region of the female part is purely frustoconical so that the barrel portion engages it in line contact. In addition the outer end of the male part is frustoconical and the female part is formed with a complementary shoulder. The inclination of the frustoconical end and shoulder surface is such that they engage each other the outer end of the male part is cammed outward. This causes the barrel portion to roll off in annular line contact on the frustoconical region of the female part as the two parts are power tightened together, forming a tight annular seal line.
Similarly German patent document No. 2,483,387 (citing German patent document Nos. 628,915 and 2,134,274 as well as French patent No. 2,178,948 and U.S. Pat. No. 2,239,942) filed Aug. 7, 1974 by H. Kaluza describes such an arrangement wherein the cone angle of the frustoconical end of the male part is smaller than that of the shoulder so that as the two are forced together they move from line to surface contact. Otherwise as in the other above-described system, this arrangement has a wholly curved barrel region both at the outer end of the male part and on the outer end of the female part to provide two more annular seal lines.
Both systems suffer from two main drawbacks. First of all any damage to the barrel portion can lead to a serious loss of seal. Since the seal is made at the barrel portion by a region of line contact, a small pit at this region can lead to a leak. Since the barrel portion is provided on the outer surface of the outer end of the pipe, it is frequently damaged somewhat before installation, so that the pipe must be discarded or the risk of a leak tolerated.
In addition such arrangements concentrate so much force in the various regions of line contact that material failure and deformation at these regions is likely. The joint can thus become weakened and/or frozen due to the flowing of the steel of the pipes at the seal lines.